This invention relates to an electric discharge machining method using an electric discharge machine of the type that cuts a workpiece by means of a wire electrode, or a so-called "wire-cut" electric discharge machine. More particularly, the invention relates to a wire-cut electric discharge machining method wherein sets of machining conditions are stored beforehand in a computer memory, one of the sets is issued as a command in response to a machining program, and a workpiece is cut by the wire electrode based on the commanded set of machining conditions.
In a wire-cut electric discharge machine, a voltage differential is established between the travelling wire electrode and a workpiece to produce an electrical discharge or spark across the intervening gap. Relative movement is established between the workpiece and the wire electrode on the basis of machining command data while the electrical discharge erodes the workpiece in small increments, thereby cutting a workpiece into the desired shape. In an electric discharge machine of this type, machining conditions change depending upon the nature and thickness of the workpiece material and the diameter of the wire electrode. Such conditions include the voltage impressed across the gap between the electrode and workpiece, the resulting peak current and average current magnitudes, the capacitance of the discharge circuit, the duty (on-off time) of the voltage pulses applied to the gap, the wire electrode tension, servo voltage, and the specific resistance of the machining fluid (dielectric) introduced into the gap. The operator must therefore take into consideration the thickness and properties of the workpiece as well as the electrode diameter and set the optimum machining conditions on an associated numerical control device prior to the start of cutting. The wire-cut electric discharge machining operation may then proceed on the basis of the set conditions.
Another important factor in electric discharge machining is that the thickness of a workpiece is not always uniform. When such is the case, it is necessary to change the machining conditions in accordance with the change in thickness. With the conventional wire-cut electric discharge machine, however, only one set of machining conditions can be set on the operator's panel. In order to changeover from one set of conditions to another set in accordance with workpiece thickness, therefore, the operator must monitor the change in thickness and, whenever a change is noted, must manually set, the proper values for impressed voltage, peak current, pulse duty cycle and wire electrode tension, etc. This is an extremely laborious task and makes unattended operation impossible to achieve unattended operation. Though it is feasible to adopt an arrangement wherein more than one set of machining conditions may be set on the operator's panel at the same time, this would entail a multiplicity of different switches and lead to an apparatus of larger size and higher cost.